1. Field of the Invention
This invention relates generally to the field of rigid disc drive data storage devices and more particularly to a limit stop for defining the range of motion of the disc drive actuator and into which can be incorporated a magnetic carriage latch in which the permanent magnet of the voice coil actuator motor is used to provide the latching force.
2. Brief Description of the Prior Art
Disc drive data storage devices have found widespread use as a cost effective and convenient mechanism for the storage of large computer programs and user generated data. Those types of disc drives known as "Winchester" disc drives incorporate rigid media, and the read/write heads "fly" above the disc surface on a thin layer of air dragged along by the spinning of the discs. An actuator mechanism is included to precisely move the read/write heads from one of a large number of circular, concentric tracks to another, in order to access any data recorded on the disc surface.
Market demands for increased storage capacity and decreased access time have lead to the almost exclusive use of voice coil motors (VCMs) to drive the actuator which positions the heads, while recent trends toward smaller and smaller form factors have lead to the preeminence of rotary VCM actuators. In a typical rotary VCM actuator, an array of permanent magnets with flux-concentrating pole pieces is fixedly mounted to a housing base or base plate, with a horizontal air gap provided which is the location of a high concentration of magnetic flux. A horizontal coil mounted to a pivot housing, which turns about a vertical axis, is suspended in the magnetic field, and an array of read/write heads is cantilevered from the side of the pivot housing opposite the coil and over the stack of discs. Controlled DC power is applied to the ends of the coil, and the polarity and magnitude of the DC voltage determines the direction and speed of the movement of the heads in accordance with the well-known Lorentz relationship.
Since VCM actuators have no inherent magnetic detent when power is removed from the disc drive, such disc drives must incorporate some sort of latching mechanism to hold the heads in a stationary relationship to the discs when power is removed, in order to prevent damaging relative movement between the heads and discs while the discs are not spinning. Similarly, some device must also be included to limit the range of motion of the actuator, should a "runaway" condition occur in the electronics controlling the coil power.
Historically, actuator latches--sometimes also referred to as carriage latches--have been of several different types, including solenoid-operated latches and magnetic latches. The main drawback of solenoid-operated latches is the power needed to operate them, and the physical size of the solenoids. Magnetic latches incorporating permanent magnets also take up significant space, and their operation can be severely influenced by the presence of the permanent magnet in the VCM of the actuator. Another common fault with magnetic latches is that they start to have an influence on the motion of the actuator even when the actuator is at a significant distance from its intended latch-up position, thus biasing the power required to move or hold the read/write heads within this "area of influence".
It is common practice to have the latch incorporate a limit stop to define one end of the range of motion of the actuator, while another mechanism is used to limit and define the other end of the range of motion.
Current industry trends toward the two-and-a-half inch and smaller form factors for disc drives have made it necessary to eliminate as many components as possible from the disc drive assembly, while minimizing the size of the remainder of components.
Clearly a need exists for a carriage latch which occupies a minimum of space within the disc drive structure, eliminates a large number of components when compared to prior art devices and yet provides adequate latching force to assure that the actuator remains latched under specified non-operating shock loads. Ideally, this latch should be integrated with the limit stops which define the range of motion of the actuator.